Proper stabilization of the head is essential for humans to carry out many of the essential activities of daily living. Throughout most of the activities in which we engage the head is held in a stereotyped position with respect to gravity. This helps to maintain the orientation of the head's special sensory receptors in space and regulates the attitude of the head on the trunk as part of overall postural control. Vestibulocollic reflexes, which utilize information from head position and velocity sensors in the otolith and semicircular canal receptors of the vestibular labyrinth to generate neck muscle activity to stabilize the head are a critical part of the head stabilization system. Our long-term goal is to determine how vestibulocollic reflexes (VCRs) contribute to head stabilization during linear, angular and combined linear-angular perturbations of the trunk in the sagittal and frontal planes. This project concentrates on the role of VCRs generated by otolith afferents and on the ways in which they interact with better known semicircular canal VCRs. We propose two series of experiments on normal human volunteers. The first will examine how the linear and angular VCRs interact with one another and with head movements generated by voluntary and visuomotor systems. This will help us understand the contexts in which VCRs contribute to maintaining head stability and the ways in which they are regulated or "gated" by other parts of the motor control system. The second series of experiments will measure the dynamic properties of otolith and canal VCRs across a wide range of frequencies and types of stimuli under conditions that minimize the contribution of visual and voluntary head movement systems. The two reflexes will be studied together during rotations or translations of the trunk with the head free to move while the VCRs will be studied in isolation during rotations or translations with the head fixed to the trunk. These experiments will help us understand the internal processing that occurs in these reflexes and the way in which their output is modified by the biomechanical properties of the head-neck system. Experimental results will be interpreted using two models. The first is a dynamic model which starts with well tested vestibuloocular reflex models and adds non-linear behavior and multiple rotation axes that characterize the head movement system. It will incorporate elements corresponding to known physiology of labyrinthine receptors and reflex pathways and will attempt to show how position, velocity and acceleration information, embedded in firing patterns of regular and irregular peripheral afferents, drives neck muscles to maintain stability of the head in space. The second is a detailed biomechanical model of the human head-neck system that quantifies the actions of all joints, muscles and passive mechanics and allows prediction of appropriate patterns of muscle activity to stabilize the head in the face of angular and linear perturbations.